Apparatus and method for transmitting a digitized signal, and a data source and data sink to implement the method

ABSTRACT

A voice signal is transmitted in an MOST network on a single channel. The width of the transmitted voice data words is preferably up to 14 bits, and each voice data word is transmitted into successive clock periods of the MOST network, in a byte that includes seven bits of the voice data word and one identifier bit.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to data transmission systems, and in particular to an apparatus and method for transmitting digitized data between components of a motor vehicle data network, such as for example a motor vehicle data network.

[0002] The transmission of digitized signals, especially in real time, often encounters the problem that the rate at which the digitized data are generated and their resolution (i.e., the number of bits of their data values) are poorly matched to a given transmission infrastructure. Many data transmission systems, especially multiplexed systems, manage their transmission capacity in the form of channels, which are characterized by an operating frequency and a word width. If this word width is less than that of a transmitted digitized channel, several channels must be provided to transmit the signal, even if the quantity of data transmitted per unit time might find room in one channel.

[0003] A typical example of such a problem is the transmission of voice signals on a Media Orientated System Transport (MOST) bus system. The MOST bus includes an optical bus system that is used in motor vehicles for the transmission of data that are not directly specific to the operation of the motor vehicle. Such data can include, for example, audio and/or video signals of a car radio or car telephone, or navigation signals for a GPS system.

[0004] The MOST specification provides for a bus system with a ring structure for synchronous data transmission with a uniform system clock pulse of 44.1 kHz. Frames are transmitted on the ring bus at this frequency, and each frame contains a plurality of bytes. These bytes, which always recur at the system cycle of 44.1 kHz, are also referred to as channels. Every data source connected to the ring bus uses a certain number of such channels to send and/or receive data.

[0005] According to the MOST specification, four bytes per frame (i.e., four channels) are specified for the transmission of audio signals. This bandwidth is necessary for high-grade stereo transmission of arbitrary audio signals. However, due to the limited frequency spectrum of the human voice, its transmission requires less bandwidth. Especially in the case of a telephone voice signal, which is transmitted monophonically, the data transmitted on the MOST bus are always pairwise the same. Thus the MOST bus is used rather inefficiently for voice transmission data.

[0006] Therefore, there is a need for an apparatus and method of efficiently transmitting data such as voice data between components of a vehicle data network.

SUMMARY OF THE INVENTION

[0007] The present invention improves the utilization of the capacity of a transmission system in the event that the operating frequency and channel width of the system do not match the word frequency and word width of the digitized signal. The data words of the digitized signal are decomposed into partial words with a smaller width than that of the channels on which they must be transmitted. Consequently, in each individual channel there remains a residual transmission capacity that makes it possible to transmit an identifier for each individual transmitted partial data word. A data sink, connected to the transmission system, uses this identifier to obtain information about the position of the respective partial data word in its original data word.

[0008] This identifier is preferably one bit (i.e., a Boolean signal). Even if the number of partial data words into which each original data word is decomposed should be greater than two, a single bit is sufficient for example to distinguish every first or every last partial data word from all the other partial data words of a data word, and thus to deliver to the sink a fixed point from which it can begin to reconstruct the original data words.

[0009] A preferred application of the present invention is a transmission system for which the ratio a of the operating frequency of the channel to the word frequency of the digitized signal is equal to two. If it is also assumed that the 8-bit channels have a width m, the present invention is well suited for economical transmission of voice signals in an MOST network.

[0010] MOST networks presently operate at an operating frequency or frame frequency of 44.10 kHz. However, since the frequency spectrum of the human voice generally does not extend beyond 11 kHz, signal, a sampling frequency of only 22 kHz is required. Therefore, sampling voice signals at 22.05 kHz which is one-half of the MOST operating frequency is acceptable.

[0011] Commercial analog-to-digital converters (ADCs) typically have a resolution of twelve or sixteen bits. The data words of a digitized signal with twelve-bit width, including an identifier, can be transmitted on two channels of a transmission system, where each channel is eight bits wide. However, part of the transmission capacity of the system would remain unutilized. Complete utilization of the transmission capacity is achieved if the data words are fourteen bits wide. Such a digitized signal can be generated by sixteen-bit converters, since the sampled sixteen bit signal can be truncated to a width of fourteen bits.

[0012] In one embodiment, a sampled sixteen bit signal is converted into partial data words each having identifier and suitable for transmission, by shifting bits 2 to 8 of the 16-bit scanning datum one bit to the right (the bits of the scanning datum are numbered 0 to 15, where bit 0 is the least significant bit). This results in a transformed data word, whose bits 15 to 9 coincide with those of the original scanning datum, whereas bits 7 to 1 respectively have the values of bits 8 to 2 of the scanning datum. Bits 8 and 0 are set to the value 1 and 0, respectively. The two bytes of the transformed data word can now be transmitted on one channel in two successive frames, such that one frame contains the more significant byte with bits 15 to 8 and another the less significant byte with bits 7 to 0. The least significant bit in each of these two bytes defines each byte as the first or second partial word.

[0013] When the transmitted data are recovered at the sink, the shift operation is reversed. The sink checks the least significant bit of each received byte to determine if the byte is the least significant or most significant byte of the transformed data word, and is thus able to reconstitute the transformed data word with the correct sequence of bytes.

[0014] The sink receives the sampled data of a voice signal at half the clock pulse rate of the MOST network. To transmit sampled data at the full MOST clock pulse rate of 44.1 kHz to a subordinate unit, the sink transmits each received data word of the voice signal twice.

[0015] To reduce garbling of the voice signal received by the subordinate unit, the sink transmits the voice signal through a low-pass filter.

[0016] A data source which is especially suitable to implement the technique of the present invention preferably includes a logic circuit to shift bits 2 to 8 of a 16-bit voice data word by one bit to the right and to enter an identifier, as the first or second partial word, in the lowest significant bit of each of the two bytes of the resulting data word.

[0017] A complementary data sink is characterized by a logic circuit to recombine two received bytes of voice data into one 16-bit-wide data word, and to shift the less significant byte of the data word by one bit to the left.

[0018] Advantageously, the apparatus and method of the present invention allows sampled voice data to be transmitted more efficiently in a MOST data network.

[0019] These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0020]FIG. 1 is a block diagram illustration of a MOST ring network;

[0021] FIGS. 2A-2E illustrate partitioning and bit manipulation of sampled data in response to processing of the invention;

[0022]FIG. 3 is a block diagram illustration of one embodiment of a data source; and

[0023]FIG. 4 is a block diagram illustration of one embodiment of a data sink.

DETAILED DESCRIPTION OF THE INVENTION

[0024]FIG. 1 is a block diagram illustration of a MOST ring network 10. The network 10 includes a ring-shaped bus 1, whose various component sections 3 link interface units 2. The interface units function as data sources or data sinks that transparently forward data that are not intended for them. Various data processing devices, such as for example, a computer of a navigation system, a radio or television receiver, or the like, or as shown here the handset 4 of a mobile telephone and its wireless transmitter/receiver unit 5 can be connected to the interface units 2. The radio, radio or television receiver, also a hands-free device of the mobile telephone can share via the bus access to an amplifier 6 with a connected loudspeaker 7. The handset 4 and the transmitter/receiver unit 5 exchange voice signals with one another and in some circumstances with the amplifier 6 via the hands-free device.

[0025] The present invention shall be discussed in the context of a data transmission from the handset 4 (i.e., a data source) to the transmitter/receiver (i.e., a data sink) 5. Of course, the present invention is not so limited and any number of transmission paths can utilize the processing of the present invention (e.g., transmissions from the handset 4 or the transmitter/receiver unit 5 to the amplifier 6).

[0026] The handset 4 delivers to its interface unit 2 a sampled voice signal preferably in the form of 16-bit-wide data words with a clock pulse rate of 22.05 MHz, corresponding to the half-frame frequency of the MOST bus. Therefore, the frequencies have a ratio of a equal to two (i.e., a=2).

[0027]FIG. 2A illustrates the sixteen-bit sampled voice signal from the handset 4. The signals include bits 0 to 15, containing the values bo to bl5, respectively, where bit 0 is the least significant bit.

[0028]FIG. 2B illustrates the sixteen-bit sampled voice signal following its shifting of bits 1 to 8 to the right by the interface unit 2 associated with the handset 2, in preparing transmitting data voice data from the handset on the bus. Notably, bits 15 to 9 are unchanged, while the contents of bits 8 to 1 are shifted to the right by one bit. The content of bit 8 is not specified, as indicated by a cross 100 in the corresponding box. Bit 8 is then set equal to one, and the content of bit 0 is overwritten with zero. The bit value bo is lost during the shift operation. FIG. 2C illustrates the resultant two bytes.

[0029] Referring to FIG. 2C, the resulting transformed data word is divided into two bytes, each of which contains a 7-bit-wide partial word of the original sampled data word in bits 7 to 1, and the identifier of the partial word as the most or least significant part of the original data word in bit 0. These two data words are transmitted by the interface unit onto the bus in successive frames. It must be specified by convention which of the partial words is transmitted first.

[0030] Following receipt of a byte transmitted onto the bus, the interface unit 2 associated with transmitter/receiver unit 5, which functions as data sink, checks the value of the bit 0 to determine if the byte is the most significant or least significant byte of the transformed data word. In virtue of the convention, it is able to reconstitute the data words in their original form. This state is symbolically shown in FIG. 4D.

[0031] The interface unit of the transmitter/receiver unit 5 receives the data transmitted on the bus as received data illustrated in FIG. 2D byte by byte. The interface unit of the transmitter/receiver unit 5 then processes the data illustrated in FIG. 2D to reconstruct the voice data. That is, referring to FIG. 2E, the interface unit shifts bits 7 to 1 of the least significant data word (i.e., contents b₈ to b₂) by one bit to the left. Significantly, the original sequence of bits b₁₅ to b₂ is illustrated in FIG. 2A is reestablished. However, bits b₁ and b₀ are lost and are replaced by zeroes as shown in FIG. 2E. In this way, the transmitter/receiver unit 5 receives a complete data word at a frequency of 22.05 kHz.

[0032] Referring again to FIG. 1, the amplifier 6 receives general audio signals on four channels of the MOST bus with the full-frame frequency of 44.1 kHz. So that the amplifier can interact with the transmitter/receiver unit 5, which delivers data words at only half the frame frequency, the interface unit 2 of the amplifier outputs each received data word of a voice signal twice to the amplifier 6. To prevent aliasing effects from the repeated output of the same value, a low-pass filter is connected between the interface unit and the amplifier 6. In many cases, utilization of such a filter requires no additional circuitry. Devices such as the amplifier 6, which require a voice signal with the full system clock pulse of 44.1 kHz, in practice generally are such as are also used to process general audio signals with a specified transmission bandwidth of four channels. As a rule, such devices have digital signal processors to implement tone controls, which can also be configured as input filter stages.

[0033]FIG. 3 is a block diagram illustration of one embodiment of a data source 30 (e.g., the interface unit associated with the handset 4). In one embodiment, the data source 30 includes an analog-to-digital converter (ADC) 9, which receives an analog voice signal and outputs 16-bit-wide sampled data at a frequency of 22.05 kHz. The 16-bit-wide output signal of the ADC 9 is divided into bits b₁₅ to b₉ and b₈ to b₂, which are stored temporarily in the seven most significant bits of two shift registers 11, 12, respectively. The least significant input of shift register 11 is set constantly to logical level one, and the least significant input of shift register 12 to a logical zero. The outputs of the shift registers 11, 12 are connected to the inputs of a multiplexer 13. In each operating cycle of the ADC, the multiplexer 13 switches through to the output 14 once the shift registers 11 and once the shift registers 12. The circuit thus outputs data bytes at a rate of 44.1 kHz, containing in alternation respectively the bits 15 to 9 and bits 8 to 2 of a digitized data word as illustrated in FIG. 2C.

[0034]FIG. 4 is a block diagram illustration of one embodiment of a data sink 40 (e.g., the interface unit associated with the amplifier 6). The data sink includes a register 21 at its input 20, to buffer arriving bytes. The data inputs of a demultiplexer 22 are connected to the seven most significant outputs of the register 21. The least significant bit output of the register 21 is connected to the control input of the demultiplexer 22. Depending on the value of the least significant byte, the go demultiplexer 22 outputs the received data to one of two registers 23, 24. The register 23 receives the most significant bits b₁₅ to b₉ of the 16-bit-wide output 25 to the data sink, while register 24 delivers the other bits b₈ to b₂. Bits b₁, b₂ are set constantly to zero. The 16-bit-wide output 25 is illustrated in FIG. 2E.

[0035] The invention has been discussed in the context of voice transmission in a MOST network. However, one of ordinary skill in the art will recognize that the invention is applicable to the transmission of arbitrary digitized signals with a word width of n bits on one or more channels (depending on the ratio of the scanning frequency to the operating frequency) of a transmission system with a width of m bits, where m is smaller than the word width n.

[0036] Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention. 

1. A method for transmitting a signal digitized with a scanning frequency, with a word width of n bits, from a data source to a data sink in a data transmission system, with an operating frequency, which has at least one channel with a width of m bits, such that n is greater then m, and the operating frequency is greater than the scanning frequency, comprising decomposing each data word of the signal into at least two partial words with a width that is less than m, and that at least two partial words obtained from one data word are transmitted on the channel together with an identifier that identifies the position of the partial word in the original data word.
 2. The method of claim 1, wherein, after transmission on at least one channel, the original data word is reconstructed from the partial data words by means of the identifier.
 3. The method of claim 1, wherein the identifier comprises one bit.
 4. The method of claim 1, wherein as many as a partial data words of one data word are transmitted on one channel if the operating frequency is at least a times the scanning frequency and less than (a-1) times, wherein a is an integer value.
 5. The method of claim 4, wherein a is equal to two.
 6. The method of claim 1, wherein n is equal to fourteen and m is equal to eight.
 7. The method of claim 6, wherein each data word is obtained by rounding to fourteen bits an analog signal scanned with a resolution of sixteen bits.
 8. The method of claim 7, wherein bits 2 to 8 of the 16-bit data word are right-shifted by one bit in order to reduce the width, and that the identifier of the byte as the first or second partial word is entered into the respective least significant bits of the resulting two bytes.
 9. The method of claim 1, wherein the digitized signal is obtained by sampling spoken speech.
 10. The method of claim 9, wherein, in the data sink, the two partial words are recombined into the original data word by shifting the least significant byte one bit to the left.
 11. The method of claim 10, wherein the two least significant bits of the recombined data word are set to a prescribed value.
 12. The method of claim 1, wherein the sink forwards a digital signal at the operating frequency by outputting each received data word several times.
 13. The method of claim 12, wherein the data sink forwards the digital signal over a low-pass filter.
 14. The method of claim 1, wherein the data transmission system is configured and arranged as a MOST network.
 15. A data source for transmitting a digitized signal with a word width of n bits in a data transmission system, which has at least one channel with a width of m bits, such that n is greater than, comprising by a logic circuit to decompose each data word into a plurality of partial words with a width less than m bits and to add an identifier to at least one of the resulting partial words, to identify the position of the partial data word.
 16. The data source of claim 15, wherein the logic circuit is designed to shift bits 2 to 8 of a 16-bit data word of the digitized signal by one bit to the right, and to enter an identifier as first or respectively second partial word, into the respective least significant bit of the two bytes of the resulting data word.
 17. A data sink to reconstruct a digitized signal with a word width of n bits, by means of data transmitted on an m bit-wide channel of a data transmission system, such that n is greater than m, comprising a logic circuit to decompose each received data value into a partial word with a width less than m bits and an identifier, and to recombine the partial words to an original data word of the digitized signal by means of the identifier.
 18. The data sink of claim 17, wherein the logic circuit is designed to combine two received data values with a width of m=8 bits to a 16-bit-wide data word, and to shift the least significant byte of the data word by one bit to the left.
 19. The data sink of claim 18, comprising means for setting the two least significant bits of the recombined data word to a specified value. 